Process aware metrology

ABSTRACT

Systems and methods for process aware metrology are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/538,699 tied “Process Aware CD Metrology,” filed Sep. 23, 2011, whichis incorporated by reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to systems and methods for inspectingand measuring structures created during the fabrication of semiconductordevices.

2. Description of the Related Art

The following description and examples are not admitted to be prior artby virtue of their inclusion in this section.

The semiconductor industry has been using optical critical dimension(CD) metrology (such as scatterometry) since about 2000, but many of thecurrent uses in high-volume manufacturing are limited to measurement ofrelatively simple shapes, usually just a grid of parallel trenches orstructures, and measurement of relatively few shape parameters such asheight (depth), CD (width) and sidewall angle.

In order to make a measurement, a model of the structure has to beconstructed. Usually cross-section electron micrographs of the structureare needed because, in most cases, the shapes cannot be determined fromtop-down images. If the shape of the structure is three-dimensional(i.e. the structure does not have a constant cross-section anydirection), then at least two perpendicular cross-sections may be neededto reveal the shape.

A model is most often constructed from simple geometric shapes thatapproximate the shape of the structure. The dimensions of these shapesare controlled by a few parameters (such as length, width, height and/orangles). When setting up the model, a decision has to be made as towhich of these dimensional parameters will be allowed to vary during themeasurement process and which will be kept constant.

Values or models of the complex refractive indices of the materials thatmake up the structure are needed. In many cases, these will be knownfrom prior experience with these materials or by measurements atunpatterned locations on the same wafer or from other wafers processedthrough the same, or similar, equipment and processes.

Once the shapes, dimensions and refractive indices are known,electromagnetic calculations can predict how light will scatter fromthat structure. Those scattering predictions can be used to model theexpected signal when an optical instrument makes a measurement of thatstructure.

The complete model (sometimes referred as a measurement recipe) is thenused to process data collected on an optical measuring tool such as areflectometer or ellipsometer in order to determine the best fittingshape parameters, which are assumed to represent the relevant dimensionsof the actual shape.

In many cases, the model may be used to precompute a library of opticalsignatures corresponding to ranges of all the dimensional parametersthat are allowed to vary. A library may speed up the measurementsignificantly when more than 2 or 3 parameters are allowed to vary.

It is also known to construct libraries of optical signatures fromexperimentally measured optical signatures collected by measuringstructures on wafers that were processed under different conditions. Insome cases, other measurement techniques, such as SEM images, are usedalso to determine some of the dimensions.

The need for cross-section images means that an accurate model cannot beconstructed for many hours or even days after the first wafers have beenprocessed because of the time needed to prepare the wafers forcross-sectioning as well as the time required for taking the images.This delay is generally not acceptable, and the cost is high. Ofteninitial measurements have to be made using models constructed beforecross-section images are available and so those models incorporate a lotof guesswork and may in not provide accurate measurements for thestructure. If the results subsequently prove to be accurate, until thecross-section images become available, there may be a lack of confidencein the results leading to delays in acting upon those results.

Two perpendicular cross-section images plus a top-down image may notsuffice to reveal all the details of complex structures made frommultiple materials. Re-entrant features, in particular, may be missedunless a cross-section happens to go through the right location.

Since cross-sections are slow and expensive to prepare, typically only afew will be prepared. These will not show all the possible variations inshapes and dimensions that can occur with normal variations inprocessing, let alone the changes that may occur when abnormalsituations arise.

As described above, the dimensions of the geometric shapes that make upthe model are controlled by a set of parameters (such as length(s),width(s), height(s) and/or angles). When setting up the recipe,decisions have to be made which dimensional parameters should be keptfixed and which should be allowed to vary during the measurementprocess. If many, or all, parameters are allowed to vary in an attemptto maximize the flexibility of the model to track process changes, themeasurement results will usually exhibit poor repeatability (and for 20or more parameters may be unstable) because the optical signal maypoorly discriminate between certain combinations of dimensional changes.But if one or more parameters are held constant when the correspondingdimensions are actually varying, then the measurement results will beinaccurate.

The process of constructing the model of the structure involves acombination of experience, guesswork and trial and error and is, atbest, a stow process that is not consistent from person to person, and,at worst, may not result in an accurate measurement.

When a library is constructed from experimental data, the library cannotbe constructed until multiple wafers have been fully processed underdifferent process conditions and the optical measurements have beenperformed on those wafers. Such a library suffers from the disadvantagesof being noisy. Firstly, there is process noise because, even for thesame process settings on the process tool, the actual processingconditions do vary with location on the wafer and from wafer to wafer.Secondly, there is necessarily noise on the optical measurements fromoptical, thermal and electrical noise sources in the instrument.Thirdly, any reference dimensional or shape measurements (from, forexample an electron micrograph or an atomic force microscope) are alsosubject to noise and systematic errors.

Accordingly, it would be advantageous to develop process aware metrologysystems and/or methods that do not have one or more of the disadvantagesdescribed above.

SUMMARY OF THE INVENTION

The following description of various embodiments is not to be construedin any way as limiting the subject matter of the appended claims.

One embodiment relates to a computer-implemented method for generatingan optical model of a structure to be measured on a semiconductor wafer.The method includes selecting nominal values and one or more differentvalues of process parameters for one or more process steps used to formthe structure on the wafer. The method also includes simulating one ormore characteristics of the structure that would be formed on the waferusing the nominal values. In addition, the method includes generating aninitial model of the structure based on results of the simulating step.The method further includes simulating the one or more characteristicsof the structure that would be formed on the wafer using the one or moredifferent values as input to the initial model. The method also includestranslating results of both of the simulating steps into the opticalmodel of the structure. In addition, the method includes determiningparameterization of the optical model based on how the one or morecharacteristics of the structure vary between at least two of thenominal values and the one or more different values. The selecting step,both simulating steps, the generating step, the translating step, andthe determining step are performed without using images of the structureas formed on a wafer and may be started before the structure is formedon any wafers. The selecting step, both simulating steps, the generatingstep, the translating step, and the determining step are performed usinga computer system.

Each of the steps of the method described above may be further performedas described herein. In addition, each of the steps of the method may beperformed using any of the system(s) described herein. Furthermore, themethod may include any other step(s) described herein.

Another embodiment relates to a non-transitory computer-readable mediumstoring program instructions executable on a computer system forperforming a computer-implemented method for generating an optical modelof a structure to be measured on a semiconductor wafer. Thecomputer-implemented method executable by the program instructionsincludes the steps of the above-described computer-implemented method.The computer-readable medium may be further configured as describedherein.

An additional embodiment relates to a system configured to generate anoptical model of a structure to be measured on a semiconductor wafer.The system includes an optical measurement subsystem configured tomeasure the structure as formed on the wafer. The system also includes acomputer subsystem configured for performing the steps of thecomputer-implemented method described above. The system may be furtherconfigured as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a flow chart illustrating one embodiment of a simulation andmodel development process;

FIG. 2 is a schematic diagram illustrating a cross-sectional view of oneexample of a structure created by etching and deposition;

FIG. 3 is a schematic diagram illustrating cross-sectional views ofdifferent examples of variations in shape of the structure shown in FIG.2 due to changes in process conditions;

FIGS. 4 a and 4 b are plots that show the results of simulating thepolarized optical reflectivity that would be measured by a spectroscopicellipsometer from the structures of FIG. 3;

FIG. 5 is a flow chart illustrating one embodiment of a librarydevelopment process;

FIG. 6 is a flow chart illustrating one embodiment for interpretingoptical critical dimension (CD) measurement results;

FIG. 7 is a flow chart illustrating one embodiment for interpretingoptical CD measurement results using TCAD;

FIG. 8 is a flow chart illustrating one embodiment for developing asimplified process model for interpreting results;

FIG. 9 is a flow chart illustrating one embodiment for designing a teststructure;

FIG. 10 is a schematic diagram illustrating a cross-sectional view ofone example of oxide thickness variation;

FIGS. 11 a, 11 b, and 11 c are plots illustrating examples of how oxideand trench can vary with process conditions;

FIG. 12 is a block diagram illustrating one embodiment of anon-transitory computer-readable medium;

FIG. 13 is a block diagram illustrating one embodiment of a system; and

FIGS. 14 and 15 are schematic diagrams illustrating side views ofvarious embodiments of an optical measurement subsystem that may beincluded in the systems described herein.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures is greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to the samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

The embodiments described herein enable optical measurement of theshapes of structures created by lithography and etching, providequantitative information to allow adjustment of the processingconditions of subsequent wafers, and can be used to determine whetherthe dimensions of the structures are within preset control limits. Theembodiments described herein also enable designing structures thatfacilitate optical measurements and simplifying and speeding up thedevelopment of optical models and measurement recipes.

One embodiment relates to a computer-implemented method for generatingan optical model of a structure to be measured on a semiconductor wafer.FIG. 1 illustrates one embodiment of a method for the simulation andmodel development. The method includes selecting nominal values and oneor more different values of process parameters for one or more processsteps used to form the structure on the wafer. The one or more processsteps may include any process steps involved in manufacturing of thewafer. For example, as shown in FIG. 1, the method includes determiningprocess parameters, as shown in step 101. The process parameters can bedetermined in any suitable manner (e.g., from a fab database). Thevalues for the process parameters may include nominal values and somevariations of the values from nominal.

The method includes simulating one or more characteristics of thestructure that would be formed on the wafer using the nominal values.For example, the method includes simulating the expected shape producedby the process, as shown in step 103. Simulating the expected shape canbe performed using the ATHENA and VICTORY Process software packages soldby Silvaco, Inc. of Santa Clara, Calif. to predict the shapes ofsemiconductor structures created by deposition, lithography and etchunder the process parameters.

The method also includes generating an initial model of the structurebased on results of the simulating step. For example, the predictedshape of a structure under nominal process conditions can be used as thenominal shape of the structure for the initial model, thus allowingmodels to be developed without waiting for images of structures. In thismanner, once the process conditions have been set, models can bedeveloped even before wafers are processed.

The method further includes simulating the one or more characteristicsof the structure that would be formed on the wafer using the one or moredifferent values as input to the initial model. For example, processsimulation software such as the ATHENA and VICTORY Process softwarepackages can be used to predict how the shapes of semiconductorstructures created by deposition, lithography and etch can vary underdifferent process conditions. In addition, the method includestranslating results of both of the simulating steps into the opticalmodel of the structure. For example as shown in step 105, the method mayinclude translating the output of process simulation. In addition, asshown in step 107, the method may include creating an optical model.

The method also includes determining parameterization of the opticalmodel based on how the one or more characteristics of the structure varybetween at least two of the nominal values and the one or more differentvalues. For example, the insight gained into how the shape may varyunder different process conditions can guide the parameterization of themodel. In one example, as etch time is increased, the curvature andslope of the side wall may both vary. In one embodiment, determining theparameterization includes selecting parameters that are included in theoptical model. In another embodiment, determining the parameterizationincludes determining parameters of the optical model that are allowed tovary. For example, if the mathematical model that describes the sidewall can be constructed using fewer parameters when constrained to theshapes that the process can produce, then fewer parameters need to beallowed to vary when making the measurement resulting in bettermeasurement repeatability and accuracy than a model with more parametersand/or poorly chosen parameters.

The selecting step, both simulating steps, the generating step, thetranslating step, and the determining step may be performed withoutusing images of the structure as formed on a wafer and before thestructure is formed on any wafers. For example, the predicted shape of astructure under nominal process conditions can be used as the nominalshape of the structure for the initial model, thus allowing models to bedeveloped without waiting for images of structures. Once the processconditions have been set, models can be developed even before wafers areprocessed.

The selecting step, both simulating steps, the generating step, thetranslating step, and the determining step are performed using acomputer system. The computer system may perform the steps as describedfurther herein.

As shown in FIG. 1, the method may also include simulating lightscattering, as shown in step 109, and developing an optical criticaldimension (CD) measurement recipe, as shown in step 111, both of whichmay be performed as described further herein.

FIG. 2 is an example of a cross-section of a test structure created on asemiconductor wafer by etching and deposition. The structure includes aseries of parallel trenches. The pattern repeats on a regular pitch (200nm in this example). The figure just shows a cross-section of a singleunit cell of that repeating structure. Isolation trenches 202 are etchedinto silicon 201 and lilted with oxide. Typically, the oxide will beplanarized after tilling. In this example, the trenches areapproximately 50 nm deep. Then, after appropriate patterning, recesses203 are etched into the silicon to a depth of about 75 nm. Aftercleaning and surface preparation, oxide film 204 is grown on the siliconsurface (in a thermal oxidation step) to a thickness of approximately 4nm. This test structure is constructed at the same time as thetransistors and other circuit structures of the semiconductor device arefabricated from the same (or a superset of the same) process steps.Typically, the test structure is a simplified shape compared with actualdevices. In this example, the test structure includes parallel trenchesthat are microns in length. It is to be understood that the shapes anddimensions of this example are illustrative and not limiting. Manydifferent shapes and dimensions could be used in practice. Typically,the test structure will incorporate important dimensions from thedevices being fabricated, so that measurements of the test structureprovide information about the dimensions of the devices. It is also tobe understood that the test structures need not be two-dimensionalstructures including parallel features with a constant cross-section.Three-dimensional shapes more closely representing the shapes of thedevices being fabricated may also be used.

FIG. 3 shows variations in shape of the structure shown in FIG. 2 due tochanges in process conditions. In particular, FIG. 3 shows a series ofcross-sections of the upper part of the structure shown in FIG. 2 fordifferent process conditions. In order to keep the figure simple, thisexample just assumes that two process parameters, an etch rate, ki, andan oxide growth time, DT, are varying. In practice, there will bemultiple process parameters that may vary for each of the process steps.The process simulation shows how the cross-sectional shape of the recessand the thickness of the oxide layer vary with process conditions. Inthis example, the simulation software predicts that the recess has across-section that is approximately a truncated ellipse in shape.Without the process simulation, an incorrect cross-section shape, suchas a ‘U’ shape, might be assumed and a less accurate model would bebuilt. With realistic assumptions about the cross-sectional shape, anappropriate parameterization of the shape can be chosen.

FIGS. 10 and 11 a, 11 b, and 11 e show more detail on how the shapesshown in FIG. 3 vary with process conditions. FIG. 10 shows a definitionof a channel length or gate length L along the curved surface of thetrench. FIG. 10 also shows how a thickness d of the oxide layer isdefined as a function of position along that curved surface as theminimum distance between the outside surface and the silicon surface. Inother words, oxide thickness, d, can be defined by the minimal distancesbetween the two interfaces of the gate oxide. A mean thickness, mean(d),and a thickness variance, variance(d), are defined in the equationsshown in this figure. FIGS. 11 a, 11 b, and 11 c show how thesedimensional parameters vary with process conditions. In particular, FIG.11 a shows the average gate thickness (in nm) as a function of etchrate, ki, for different oxide growth times (TD). FIG. 11 b shows thevariance of gate thickness (in nm) as a function of etch rate, ki, fordifferent oxide growth times (TD). FIG. 11 c shows the gate length (innm) as a function of etch rate, ki, for different oxide growth times(TD).

In some embodiments, the method includes determining a relationshipbetween the one or more characteristics and the nominal and one or moredifferent values, measuring the structure as formed on the wafer usingan optical measurement technique, using results of the measuring todetermine the one or more characteristics of the structure as formed onthe wafer, and determining values of the process parameters used to formthe structure on the wafer using the one or more determinedcharacteristics in combination with the relationship. For example,simulations and analyses such as those shown in FIGS. 3, 10, and 11 a,11 b, and 11 c can be used to interpret the results of opticalscatterometry measurements of the dimensions of the structure in termsof process conditions or changes in process conditions. Givenmeasurements of, for example, average oxide (gate) thickness and gatelength, relationships such as those plotted in FIGS. 11 a, 11 b, and 11c can be used to infer process conditions.

In one embodiment, both simulating steps include simulating the one ormore characteristics as a function of position across the structure, andthe initial model and the optical model are created to includemathematical functions that define variations in at least one of the oneor more characteristics as a function of the position across thestructure. For example, the oxide thickness variation will be differentfor different process conditions. In one example, a careful examinationof the plots in FIG. 3 show that the oxide thickness, d, varies withposition along the curved surface of the trench. Plots can be generated,each corresponding to one of the sets of process conditions shown inFIG. 3. Each plot could show the thickness of the oxide as a function ofposition along the surface of the trench from the left end of the lefttrench through to the right end of that trench. Each plot would showthat the oxide thickness is not constant along the surface of thetrench. This information can be used to correctly parameterize the modelof the shape. Without the simulation results, the person creating themodel would most likely just assume constant thickness for the oxidelayer. However, in the embodiments described herein, a mathematicalcurve may be used to approximate the shape of the oxide layer and may becharacterized by a small number of (ideally 2 or 3) varying parameters.If the curve has too many parameters, the measurement repeatability willbe poor.

In another embodiment, the method includes simulating results of opticalmeasurements of the structure that would be formed on the wafer usingthe nominal values and the one or more different values and determiningwhich parameters of the optical measurements are more sensitive tochanges in values of the process parameters than other parameters of theoptical measurements. In one such embodiment, the method includesdetermining the parameters of the optical measurements that will be usedto measure the structure as formed on the wafer based on the parametersof the optical measurements that are more sensitive to the changes inthe values of the process parameters than the other parameters of theoptical measurements. For example, FIGS. 4 a and 4 b show the results ofsimulating the polarized optical reflectivity that would be measured bya spectroscopic ellipsometer from the structures of FIG. 3 expressed asthe ellipsometric parameters tan Ψ and cos Δ. In this manner, FIGS. 4 aand 4 b show the optical CD spectra from process variations. A finiteelement model of the periodic structure was used to compute the opticalreflectivity. Such a model can be built using, for example, commerciallyavailable software such as COMSOL Multiphysics (COMSOL AB, Stockholm,Sweden) or JCMsuite (JCMwave GmbH, Berlin, Germany). These curves showwhich process changes can be detected most easily and which wavelengthsare sensitive to which process changes. In this example, the curvesseparate into three families according to the three different etchrates, showing that, in this example, etch rate changes of the magnitudesimulated can easily be detected. These curves also show whichwavelength ranges have the most sensitivity to process parameterchanges. For example, it can be seen in FIGS. 4 a and 4 b that thewavelength range from approximately 650 nm to 750 nm has goodsensitivity to oxide growth time (TD) for the highest etch rate(ki=0.4), but lower sensitivity to the oxide growth time for the toweretch rates. Although this example shows the optical reflectivity beingcomputed as a function of wavelength over a wide range of wavelengthsfor a narrow range of angles, it is to be understood that thereflectivity can be computed as a function of angles over wide ranges ofangles of incidence and/or azimuthal angles for one, or few,wavelengths. It is also to be understood that the reflectivity can becomputed for a variety of different polarization states includingunpolarized incident radiation, linearly polarized incident radiation,and elliptically polarized incident radiation. The reflectivity of thestructure may be calculated as ellipsometric parameters such as tan Ψand cos Δ as shown in FIGS. 4 a and 4 b, as elements, or combinations ofelements, of the Jones matrix or Mueller matrix, or as otherrepresentations of the polarized or unpolarized reflectivity.

Other algorithms besides finite element methods may used to compute theoptical reflectivity. These algorithms include the rigorous coupled wavealgorithm (RCWA) as described, for example, in U.S. Pat. Nos. 5,963,329to Conrad et al. and 6,608,690 to Niu et al. Other algorithms that canbe used include those using Green's functions, such as those describedin U.S. Pat. No. 7,038,850 to Chang et al., and finite differencemethods such as those described in U.S. Pat. No. 7,106,459 to Chu. Allof these patents are incorporated by reference as if fully set forthherein.

In one embodiment, the method includes generating a library of opticalscatterometry signatures based on the one or more characteristics of thestructure that would be formed on the wafer using the nominal values andthe one or more different values. For example, the method may includeconstructing an optical CD library for measurement of the shape of astructure on a wafer. In addition, the method may include generating alibrary of optical scatterometry signatures to speed up an optical CDmeasurement. A series of process simulations are performed for anexpected range of process parameter variations to generate a series ofexpected shapes. For example, the method may include simulating theshapes generated by a process operating on a wafer for multiplecombinations of different process parameters. In one such example, foran etch process, the simulation may include simulating the expectedshapes for a range of RF power levels, etch wafer bias voltages, etchtimes, wafer temperatures, gas flow rates, or some combination thereof.In one embodiment, the one or more different values include maximum andminimum values for one of the process parameters. For example, thesimulation may include simulating the nominal values of all theseprocess parameters and various combinations of maximum and minimumvalues of the process parameters. The set of different shapes from thissimulation may then be used to calculate the optical signatures ofscattering for the corresponding process conditions. A library may thenbe constructed using results of the simulations.

As an illustration, simulated optical responses, such as those shown inFIGS. 4 a and 4 b, which correspond to a range of process conditions,may be used to create a library. Such a process is illustrated by theflow chart of FIG. 5. In particular, FIG. 5 is a flow chart for alibrary development process. As shown in step 501 of FIG. 5, the methodmay include specifying the nominal and maximum and minimum values forprocess parameters relating to one or more process steps. As describedabove, simulations of the structures to be measured are made for theprocess steps and different combinations of process conditionsincluding, but not limited to, nominal process conditions and expectedmaximum and minimum values of different process parameters, as shown inFIG. 503. In some embodiments, all combinations of the process conditionvalues may be simulated, as in the example shown in FIGS. 4 a and 4 bfor two varying process conditions where all nine combinations ofnominal, minimum and maximum values are simulated. In other embodiments,particularly where there are many varying process conditions, a subsetof the possible combinations of process parameter values are simulated.The process conditions simulated need not be limited to nominal, minimumand maximum values. Other values lying between nominal and one of theextreme values might also be used in the simulation. For example, asshown in FIGS. 4 a and 4 b, there are relatively large changes in theoptical signatures for changes in the etch rate parameter, ki. In such acase, it might be useful to include additional values of ki in thesimulation, such as 0.25 and 0.35.

As shown in step 505, the method may also include translating simulatedprofiles into optical models. In addition, the method may includesimulating light scattering from those optical models, as shown in step507. The method may also include constructing a library relating opticalscattering to process parameters, as shown in step 509. The method mayfurther include using the library to measure process parameters ofdevices on wafers by analysis of measured optical scattering, as shownin step 511. In addition, the method may include reporting measurementresults as process parameters, as shown in step 513. All of these stepsmay be performed as described further herein.

Examples of different methods of constructing and using libraries can befound in U.S. Pat. Nos. 5,607,800 to Ziger, 5,867,276 to McNeil et al.,5,963,329 to Conrad et al., 7,280,229 to Li et al., 7,312,881 toShehegrov et al., 7,831,528 to Doddi et al., and 7,859,659 to Xu et al.,all of which are incorporated by reference as if fully set forth herein.The library may include the simulated optical scattering signatures or amachine learning system, neural network, or statistical process trainedon the simulated optical scattering. In one embodiment, generating thelibrary includes storing the optical scatterometry signatures calculatedfor the nominal values and the one or more different values. In anotherembodiment, generating the library includes training software on theoptical scatterometry signatures calculated for the nominal values andthe one or more different values. For example, a library used foroptical scatterometry may be based on storing optical signaturescalculated for different process parameters or may be based on a machinelearning system or neural network trained on those calculated opticalsignatures. Creation of the library may include performing statisticalanalysis on the optical signatures such as principal component analysisto reduce the amount of data that has to be stored without losingsignificant accuracy or sensitivity. The library may be used withinterpolation when determining the process parameters that best fit themeasured optical signal. Generating the library as described above isperformed without measuring the structure as formed on any wafers.Compared with constructing a library from experimentally determinedoptical signatures measured from wafers processed under differentconditions, the embodiments described herein have the advantage ofresulting in a less noisy measurement because no experimental noise(whether due to process variations, due to noise in the optical signal,or due to noise and errors in reference measurements) is incorporatedinto the library.

In one embodiment, the method includes determining one or morecharacteristics of the structure as formed on a wafer using an opticalmeasuring technique and determining one or more values of one or more ofthe process parameters used to form the structure on the wafer based onthe one or more characteristics of the structure as formed on the wafer.In one such embodiment, the method also includes altering one or moreparameters of a process tool based on the one or more determined valuesof the one or more of the process parameters. For example, the methodmay include using process simulation software such as ATHENA to helpinterpret the results of optical shape measurements. When one or moredimensions or shape parameters vary away from their nominal values,process simulation software can be used to determine which processparameters or conditions may have caused that change and can guide theappropriate corrective action or adjustment of the process chamber orprocess tool.

A flow chart showing one way to implement this is shown in FIG. 6. Inparticular, FIG. 6 is a flow chart for interpreting optical CDmeasurement results. As shown in step 601, the method includesperforming optical CD measurements on a structure on a wafer. As shownin step 603, the method also includes determining shapes and dimensions.In addition, as shown in step 605, the method includes using processsimulation software to determine the process conditions that resulted inthose shapes and dimensions. The method further includes adjustingprocess conditions or performing service on the process tool, as shownin step 607.

In one embodiment, the method includes determining one or morecharacteristics of the structure as formed on a wafer using an opticalmeasuring technique and determining one or more characteristics of adevice that will be formed on the wafer and that will include thestructure based on the one or more characteristics of the structure. Inanother embodiment, the method includes determining one or morecharacteristics of the structure as formed on a wafer using an opticalmeasuring technique, determining one or more values of one or more ofthe process parameters used to form the structure on the wafer based onthe one or more characteristics of the structure as formed on the wafer,and determining one or more characteristics of a device that will beformed on the wafer and that will include the structure based on the oneor more determined values of the one or more of the process parameters.For example, the method may include using TCAD (transistorcomputer-aided design) software such as ATLAS or VICTORY Device sold bySilvaco, Inc. to interpret the results of optical measurements ofdimensions and/or shapes and/or process conditions of device structuresto determine whether or not the final devices are expected to performwithin specification.

FIG. 7 is a flow chart for interpreting optical CD measurement resultsusing TCAD. As shown in step 701, the method may include performingoptical CD measurements on a structure on a wafer. In addition, as shownin step 703, the method includes determining shapes, dimensions, and/orprocess conditions of the structure. For example, the opticalmeasurements can be used to quantify the actual dimensions and shapes atdifferent locations on a wafer or the process conditions that generatedthose shapes. The actual device performance can be modeled based onthose shapes, dimensions, process conditions, or some combinationthereof. For example, as shown in step 705, the method may include usingWAD software to determine expected device performance assuming, wherenecessary, appropriate values for later process steps. As shown in step707, the method may also include, based on predicted device performance,allowing the wafer to continue processing or scrapping the wafer. Forexample, if the expected device performance is within specification, thewafers can continue to subsequent processing steps. If the expecteddevice performance is not within specification, then the wafers can bescrapped to avoid the expense of the subsequent process steps andchanges can be made to the appropriate process tool so that subsequentwafers will be within specification. In addition, as shown in step 707,the method may include making adjustments to a process tool if thedevice performance is too far from the desired performance. For example,in some cases where the expected device performance is withinspecification but is close to a limit of that specification, processconditions may be adjusted to make subsequent wafers yield devicescloser to the desired specification. The desired specification mayinclude properties related to the device speed, the device powerconsumption, the memory retention time, the memory reliability or otherimportant transistor characteristics such as threshold voltage, leakagecurrent, and saturated continuous drain current (Idsat).

Continuous drain current vs. gate source voltage (Id-Vgs) curves can beplotted for transistors constructed using the different processconditions shown in FIG. 3. In order to compute the transistorproperties, it is necessary to assume nominal (or actual measured)conditions for all the other process steps including implants and sourceand drain contacts.

one embodiment, the method includes determining two or morecharacteristics of the structure as formed on a wafer using an opticalmeasuring technique, determining one or more characteristics of a devicethat wilt be formed on the wafer and that will include the structurebased on a combination of the two or more characteristics of thestructure as formed on the wafer, and determining if the one or morecharacteristics of the device will be out of specification for the oneor more characteristics of the device. For example, in traditionalprocess control, limits are set individually on measured parameters suchas width, height, depth, slope and undercut. Typically, the limits haveto be set so that any combination of parameters within the limits willresult in devices that are in specification. But the performance of thedevices is determined by combinations of multiple parameters. If theperformance of the devices is just out of specification when allparameters are at their limit values, then there will be combinations ofparameters where some are just outside their limits and others arewithin limits that will not result in out-of-specification devices. Byusing TCAD modeling to predict the device performance, higher yields maybe obtained because certain combinations of parameters that are expectedto yield within-specification devices do not need to be rejected basedon a single parameter being outside of a fixed limit. Some parametersmay continue to be monitored based on upper and lower limits as therecan be factors other than just device performance that are alsoimportant such as compatibility with subsequent process steps.

In one embodiment, the method includes simulating, using results of bothof the simulating steps and a first model, one or more characteristicsof a device that will be formed on the wafer using the nominal and oneor more different values of the process parameters and that will includethe structure and generating a second model that is simpler than thefirst model and that describes the one or more characteristics of thedevice as a function of the results of both of the simulating steps. Forexample, FIG. 8 shows one embodiment of a method for developing asimplified process model for interpreting results. As shown in FIG. 8,the method may include running TCAD simulations for multiplecombinations of device dimensions or process conditions within processwindows, as shown in step 801. For example, since the TCAD computationof the expected device performance given shape and dimensionalinformation may be slow, in some embodiments, multiple TCAD simulationsfor different shapes and dimensions are run. As shown in step 803, themethod includes constructing a simplified model of how key deviceperformance characteristics vary with device dimensions or processconditions. For example, the results may be used to train a neuralnetwork or machine learning system or other kind of simplified model ofexpected device performance as a function of shape and dimensions for alimited range of shapes and dimensions. The method also includes makingoptical CD measurements of structure dimensions and/or shape and/orprocess conditions, as shown in step 805. That simplified model can thenbe used to quickly analyze the results of measurements on specificwafers to determine when the devices are likely to be in or out ofspecification. For example, as shown in step 807, the method may includeusing the simplified model to predict expected device performance basedon measurement results. Even if that simplified model is not as accurateas the full TCAD simulation, it may be accurate enough that the chancesof misclassifying a wafer are acceptably small. Besides neural networksand machine learning systems, other simplified models might bepolynomials such as linear, quadratic or cubic functions of theparameters, or might use other mathematical functions of the parameters.

In another embodiment, the method includes generating a test structuredesign based on results of both of the simulating steps such that one ormore characteristics of a test structure as formed on the wafer inaccordance with the test structure design are sensitive to changes invalues of one or more, but not all, of the process parameters. Forexample, the methods described herein may include using processsimulation software such as ATHENA to design test structures that areparticularly sensitive to changes in specific process parameters orprocess conditions of interest so as to make those changes easier todetect.

In some embodiments, the method includes generating first and secondtest structure designs based on results of both of the simulating stepssuch that one or more characteristics of a first test structure asformed on the wafer in accordance with the first test structure designare sensitive to a first of the process parameters but not a second ofthe process parameters and such that one or more characteristics of asecond test structure as formed on the wafer in accordance with thesecond test structure design are sensitive to the second of the processparameters but not the first of the process parameters. For example, twoor more different test structures may be designed no that each isparticularly sensitive to changes in a different subset of the processparameters or conditions such that, in combination, the two or morestructures are sensitive to changes in all the process parameters ofinterest. After a wafer has been processed, the two or more teststructures may be measured by optical CD metrology in order to determineif the process parameters are in control. Two or more differentstructures may be particularly useful when many different processparameters may be changing and from a single structure it may bedifficult to separate the effects of one process parameter changing fromanother.

In one embodiment, the method includes generating a test structuredesign based on results of both of the simulating steps such thatoptical measurements of one or more characteristics of a test structureformed on the wafer in accordance with the test structure design aresensitive to changes in the one or more characteristics of the teststructure. For example, in order to design test structures as describedherein, the method may include using process simulation software incombination with predictions of the optical scattering from teststructures. FIG. 9 illustrates a process for designing a test structure.

As shown in step 901, the method may include using TCAD modeling of theperformance of devices for a range of different dimensions such asheights, depths, widths, lengths, side-wall angles, etc. to determinehow much variability of the shape is allowed (e.g., the acceptable rangeof dimensions and shapes) given the required range of device performancethat is acceptable. This analysis will also identify which dimensions orshape parameters are most critical to device performance and so mightneed to be monitored closely.

As shown in step 903, the method includes designing an idealized teststructure that is consistent with the process design rules and whichincorporates some or all of the critical dimensions or shape parametersidentified in step 901. Other aspects of the shape of the test structuremay be simplified compared with the actual devices.

As shown in step 905, the method includes using process simulationsoftware such as ATHENA or VICTORY to predict the expected shape of thetest structure as a result of the etch, deposition, and other processesused.

As shown in step 907, the method includes performing electromagneticsimulations of the test structure response to light (e.g., to determinethe electric fields in the structure under different illuminationconditions). The electromagnetic simulation may be performed asdescribed in copending U.S. patent application Ser. No. 13/164,398 byDziura et al., filed on Jun. 20, 2011, which is incorporated herein byreference in its entirety. If the electric fields are strong near thecritical features and dimensions under at least some of the measurementconditions, then the proposed test structure may have good sensitivityto changes in those features or dimensions. If the electric field isweak near a critical feature or dimension, then the sensitivity to thatfeature or dimension is likely to be poor, and the test structure designshould be modified. This check is performed at step 910 in which it isdetermined if it is possible to get good measurement sensitivity tocritical dimensions or shape parameters. If the electric field is weak,then one or more dimensions of the proposed structure are modified atstep 912 and the simulations in step 905 are repeated. Dimensions thatmight be changed include non-critical dimensions such as the pitch ofthe repeating structure. Any new dimensions must be consistent with thedesign rules.

Step 907 may also include simulating far fields as well as, or insteadof, near fields. If a change in a dimension or shape parameter producestoo small a change in far fields (relative to system noise levels), thenthe measurement sensitivity to that change will be poor. If the changein the far field is larger than noise and errors in the measurement,then the sensitivity to that change will be good. As shown in step 920,the method may also include designing the test structure and determiningthe measurement mode to be used for the test structure. For example,step 920 may include simulating multiple different illumination and/ordetection conditions to determine which of several possible measurementmodes has the best sensitivity. In some cases, a combination ofmeasurement modes may be used, because the combination may havesensitivity to more dimensions and parameters of interest than anyindividual mode.

Test structures designed in accordance with embodiments described hereinmay be placed in the scribe lines between the die on a semiconductorwafer or may be placed in the die in regions between active circuitstructures in the die.

Although ATHENA and VICTORY Process are mentioned as examples of theprocess simulation software that may be used in embodiments describedherein, it is to be understood that any other process simulationsoftware could be substituted. For lithography process steps, alithography simulator such as PROLITH sold by KLA-Tencor Corp.,Milpitas, Calif., or SIGMA-C sold by Synopsys, Inc., Mountain View,Calif., may be used.

All of the methods described herein may include storing results of oneor more steps of the methods in a storage medium. The results mayinclude any of the results described herein and may be stored in anymanner known in the art. The storage medium may include any suitablecomputer-readable storage medium known in the art. After the resultshave been stored, the results can be accessed in the storage medium andused by any of the method or system embodiments described herein,formatted for display to a user, used by another software module,method, or system, etc. Furthermore, the results may be stored“permanently,” “semi-permanently,” temporarily, or for some period oftime.

FIG. 12 illustrates one embodiment of non-transitory computer-readablemedium 1200 storing program instructions 1202 executable on computersystem 1204 for performing a computer-implemented method for generatingan optical model of a structure to be measured on a semiconductor wafer.The method for which program instructions 1202 are executable oncomputer system 1204 may include any step(s) of any method(s) describedherein. In some embodiments, computer system 1204 may be a computersystem of an optical measurement system as described further herein. Insome alternative embodiments, the computer system may be connected tothe optical measurement system by a network. However, in otherembodiments, computer system 1204 may not be coupled to or included inan optical measurement system. In some such embodiments, computer system1204 may be configured as a stand alone computer system.Computer-readable medium 1200, program instructions 1202, and computersystem 1204 may be further configured as described herein.

Program instructions 1202 implementing methods such as those describedherein may be stored on computer-readable medium 1200. Thecomputer-readable medium may be a storage medium such as a read-onlymemory, a random access memory, a magnetic or optical disk, a magnetictape, or other non-transitory computer-readable medium.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using ActiveX controls, C++ objects, C#,JavaBeans, Microsoft Foundation Classes (“MFC”), or other technologiesor methodologies, as desired.

The computer system may include any suitable computer system known inthe art. For example, computer system 1204 may take various forms,including a personal computer system, mainframe computer system,workstation, image computer, parallel processor, or any other deviceknown in the art. In general, the term “computer system” may be broadlydefined to encompass any device having one or more processors, whichexecutes instructions from a memory medium.

Another embodiment relates to a system configured to generate an opticalmodel of a structure to be measured on a semiconductor wafer. Forexample, as shown in FIG. 13, the system includes optical measurementsubsystem 1300 configured to measure the structure as formed on thewafer. For example, any of the above methods may be used with an opticalmetrology system to perform the measurements. That optical metrologysystem might include an ellipsometer, a polarized reflectometer, onunpolarized reflectometer, a beam-profile reflectometer, or somecombination thereof. FIG. 1 of U.S. Pat. No. 5,608,526 to Piwonka-Corleet al., which is incorporated by reference as if fully set forth herein,shows one example of a spectroscopic ellipsometer that may used with themethods and test structures described herein. FIG. 16 of U.S. patentapplication Ser. No. 13/164,398, which is incorporated by reference asif fully set forth herein, shows a beam profile reflectometer suitablefor implementing embodiments described herein.

Examples of systems that could be used to measure the diffraction beamdata or signals for use with the embodiments described herein aredescribed in U.S. Pat. Nos. 6,278,519 to Rosencwaig et al., 6,611,330 toLee et al, and 6,734,967 to Piwonka-Corle et al., all of which areincorporated herein by reference in their entirety. These three patentsdescribe metrology systems that may be configured with multiplemeasurement subsystems, including one or more of a spectroscopicellipsometer, a single-wavelength ellipsometer, a broadbandreflectometer, a MTV reflectometer, a broadband polarized reflectometer,a beam-profile reflectometer, and a beam-profile ellipsometer. Thesemeasurement subsystems may be used individually, or in combination, tomeasure the reflected or diffracted beam from films and patternedstructures. The signals collected these measurements may be analyzed todetermine parameters of structures on a semiconductor wafer and/or inferprocess conditions in accordance with embodiments described herein.Embodiments described herein may be used to predict the response andsensitivity of one or more different subsystems such as those justlisted to changes in process conditions for a specific structure inorder to determine which subsystem is best for a particular measurement.

More information on how beam-profile reflectometers and ellipsometerscan be used for scatterometry measurements can be found in U.S. Pat.Nos. 6,479,943 to Opsal et al., 6,678,046 to Opsal, 6,813,034 toRosencwaig et al., and 7,206,070 to Opsal, all of which are incorporatedby reference as if fully set forth herein.

In general, the optical measurement subsystem may be configured as aspectroscopic optical measuring instrument for measuring the shape of astructure. An exemplary embodiment is shown in FIG. 14. Semiconductorwafer 1412 may include silicon substrate 1412 a, film 1412 b on thesubstrate, and structure 1412 c on the film. XYZ stage 1414 is used formoving the wafer in the horizontal XY directions. Stage 1414 may also beused to adjust the z height of wafer 1412. The instrument may includebroadband light source 1422 that is configured to generate light havinga plurality of wavelengths. That light may be directed by fiber optic1424 to an ellipsometer illuminator 1426, then through polarizer 1428,to optics (not shown) that focus tight 1430 on structure 1412 c, tooptics (not shown) that collect the light reflected from the structure,through analyzer 1432, to spectrometer 1434 that detects the reflectedlight as a function of wavelength white the polarizer 1428 or analyzer1432 is rotated. This exemplary embodiment may also include aspectroscopic reflectometer that uses lens 1423 to focus and directlight from light source 1422 through optional polarizer 1472 to beamsplitter 1452. Objective lens 1454 focuses light onto the structure 1412c and collects light reflected from the structure and directs that lightback to spectro-reflectometer 1460 that detects the light as a functionof wavelength. This subsystem may further include a focusing and patternrecognition subsystem 1464, that receives light via beam splitter 1462.The optical measurement subsystem may also include computer 1440, whichmay be configured as described herein. This exemplary embodiment is notintended to be limiting, but rather to illustrate some of the possibleelements and features of a spectroscopic optical measurement subsystemsuitable for use in certain embodiments described herein. Each of theseelements may be arranged in any suitable configuration in the opticalmeasuring instrument and may include any suitable elements known in theart. The system shown in FIG. 14 may be further configured as describedin U.S. Pat. No. 6,483,580 to Xu et al., which is incorporated byreference as if fully set forth herein.

In another example, the optical measurement subsystem may be configuredas an angle-resolved optical measuring instrument configured formeasuring the shape of a structure as illustrated by 1550 in FIG. 15.The instrument may include light source 1552 generating polarized lightbeam 1554. Preferably this light has a bandwidth of approximately 10 nmor less. In some embodiments; the source 1552 is capable of outputtingbeams of different wavelengths. Part of this beam is reflected from beamsplitter 1555 and directed to objective lens 1558 that focuses the lightonto structure 1506 on wafer 1508 in a spot size of less than about 10μm in each direction. The part of beam 1554 that is not reflected fromthe beam splitter is directed to beam intensity monitor 1557. The beammay, optionally, pass through quarter-wave plate 1556 before theobjective lens 1558. The light reflected from structure 1506 iscollected by objective lens 1558. After the beam-splitter, the reflectedbeam 1560 may optionally pass through a quarter-wave plate at location1559 as an alternative to location 1556. A polarizer or polarizing beamsplitter 1562 directs one polarization state of the reflected beam 1560to detector 1564, and, optionally, directs a different polarizationstate to an optional second detector 1566. Detectors 1564 and 1566detect the reflected light as a function of angle of incidence andazimuth angle. The diffraction beam data 1514 from the detector(s) istransmitted to profile application server 1516 along with beam intensitydata 1570. The profile application server 1516 may compare the measureddiffraction beam data 1514 after normalization or correction by beamintensity data 1570 against a library 1518 of simulated diffraction beamdata representing varying combinations of critical dimensions of thetarget structure and resolution. Each of these elements may be arrangedin any suitable configuration in the optical measuring instrument andmay include any suitable elements known in the art. The opticalmeasuring instrument shown in FIG. 15 may be further configured asdescribed in U.S. patent application Ser. No. 13/164,298 to Dziura etal., filed Jun. 20, 2011, which is incorporated by reference as if fullyset forth herein.

The system also includes computer subsystem 1302 configured forperforming the steps of the methods described above. Computer subsystem1302 may be further configured as described above with respect tocomputer system 1204. For example, the computer subsystem may beconfigured to process output responsive to the reflected light detectedby the spectroscopic or angle-resolved optical measuring instrumentdescribed above and to report a process parameter, a parameter, or anexpected device performance parameter of the structure. The computersubsystem and the system may be further configured as described herein.

U.S. Pat. No. 8,090,558 to Dziura and U.S. patent application Ser. Nos.12/841,932 to Ferns et al., filed on Jul. 22, 2010, and 61/555,108 toYoo et al. filed on Nov. 3, 2011, are incorporated by reference as iffully set forth herein. The embodiments described herein may be furtherconfigured as described in this patent and these patent applications.

The embodiments described herein provide several advantages over othercurrently used systems and methods. For example, the time required todevelop scatterometry models is reduced compared with the currentapproach. The resulting model will more accurately represent the shapeof the structures on the wafer. Measurement results can be quicklyinterpreted in terms of process conditions. Measurement results canquickly be interpreted in terms of the expected device performance. Moreaccurate disposition of wafers can be made based on the expected deviceperformance instead of occasionally discarding wafers that wouldactually have a useful yield or passing wafers for subsequent processsteps even though the device performance is poor. Test structures can bedesigned which make the metrology more sensitive to changes in a processthereby allowing more precise control of the process.

Further modifications and alternative embodiments of various aspects ofthe invention may be apparent to those skilled in the art in view ofthis description. For example, systems and methods for process awaremetrology are provided. Accordingly, this description is to be construedas illustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art alter having the benefit of this description of theinvention. Changes may be made in the elements described herein withoutdeparting from the spirit and scope of the invention as described in thefollowing claims.

What is claimed is:
 1. A computer-implemented method for generating anoptical model of a structure to be measured on a semiconductor wafer,comprising: selecting nominal values and one or more different values ofprocess parameters for one or more process steps used to form thestructure on the wafer; simulating one or more characteristics of thestructure that would be formed on the wafer using the nominal values;generating an initial model of the structure based on results of saidsimulating; simulating the one or more characteristics of the structurethat would be formed on the wafer using the one or more different valuesas input to the initial model; translating results of both of thesimulating steps into the optical model of the structure; anddetermining parameterization of the optical model based on how the oneor more characteristics of the structure vary between at least two ofthe nominal values and the one or more different values, wherein theselecting step, both simulating steps, the generating step, thetranslating step, and the determining step are performed without usingimages of the structure as formed on a wafer and before the structure isformed on any wafers, and wherein the selecting step, both simulatingsteps, the generating step, the translating step, and the determiningstep are performed using a computer system.
 2. The method of claim 1,wherein determining the parameterization comprises selecting parametersthat are included in the optical model.
 3. The method of claim 1,wherein determining the parameterization comprises determiningparameters of the optical model that are allowed to vary.
 4. The methodof claim 1, further comprising determining a relationship between theone or more characteristics and the nominal and one or more differentvalues, measuring the structure as formed on the wafer using an opticalmeasurement technique, using results of the measuring to determine theone or more characteristics of the structure as formed on the wafer, anddetermining values of the process parameters used to form the structureon the wafer using the one or more determined characteristics incombination with the relationship.
 5. The method of claim 1, whereinboth simulating steps comprise simulating the one or morecharacteristics as a function of position across the structure, andwherein the initial model and the optical model are created to includemathematical functions that define variations in at least some of theone or more characteristics as a function of the position across thestructure.
 6. The method of claim 1, further comprising simulatingresults of optical measurements of the structure that would be formed onthe wafer using the nominal values and the one or more different valuesand determining which parameters of the optical measurements are moresensitive to changes in values of the process parameters than otherparameters of the optical measurements.
 7. The method of claim 6,further comprising determining the parameters of the opticalmeasurements that will be used to measure the structure as formed on thewafer based on the parameters of the optical measurements that are moresensitive to the changes in the values of the process parameters thanthe other parameters of the optical measurements.
 8. The method of claim1, further comprising generating a library of optical scatterometrysignatures based on the one or more characteristics of the structurethat would be formed on the wafer using the nominal values and the oneor more different values.
 9. The method of claim 8, wherein the one ormore different values comprise maximum and minimum values for one of theprocess parameters.
 10. The method of claim 8, wherein generating thelibrary comprises storing the optical scatterometry signaturescalculated for the nominal values and the one or more different values.11. The method of claim 8, wherein generating the library comprisestraining software on the optical scatterometry signatures calculated forthe nominal values and the one or more different values.
 12. The methodof claim 8, wherein generating the vary is performed without measuringthe structure as formed on any wafers.
 13. The method of claim 1,further comprising determining one or more characteristics of thestructure as formed on a wafer using an optical measuring technique anddetermining one or more values of one or more of the process parametersused to form the structure on the wafer based on the one or morecharacteristics of the structure as formed on the wafer.
 14. The methodof claim 13, further comprising altering one or more parameters of aprocess tool based on the one or more determined values of the one ormore of the process parameters.
 15. The method of claim 1, furthercomprising determining one or more characteristics of the structure asformed on a wafer using an optical measuring technique and determiningone or more characteristics of a device that will be formed on the waferand that will include the structure based on the one or morecharacteristics of the structure as formed on the wafer.
 16. The methodof claim 1, further comprising determining one or more characteristicsof the structure as formed on a wafer using an optical measuringtechnique, determining one or more values of one or more of the processparameters used to form the structure on the wafer based on the one ormore characteristics of the structure as formed on the wafer, anddetermining one or more characteristics of a device that will be formedon the wafer and that will include the structure based on the one ormore determined values of the one or more of the process parameters. 17.The method of claim 1, further comprising determining two or morecharacteristics of the structure as formed on a wafer using an opticalmeasuring technique, determining one or more characteristics of a devicethat will be formed on the wafer and that will include the structurebased on a combination of the two or more characteristics of thestructure as formed on the wafer, and determining if the one or morecharacteristics of the device will be out of specification for the oneor more characteristics of the device.
 18. The method of claim 1,further comprising simulating, using results of both of the simulatingsteps and a first model, one or more characteristics of a device thatwill be formed on the wafer using the nominal and one or more differentvalues of the process parameters and that will include the structure andgenerating a second model that is simpler than the first model and thatdescribes the one or more characteristics of the device as a function ofthe results of both of the simulating steps.
 19. The method of claim 1,further comprising generating a test structure design based on resultsof both of the simulating steps such that one or more characteristics ofa test structure as formed on the wafer in accordance with the teststructure design are sensitive to changes in values of one or more, butnot all, of the process parameters.
 20. The method of claim 1, furthercomprising generating first and second test structure designs based onresults of both of the simulating steps such that one or morecharacteristics of a first test structure as formed on the wafer inaccordance with the first test structure design are sensitive to a firstof die process parameters but not a second of the process parameters andsuch that one or more characteristics of a second test structure asformed on the wafer in accordance with the second test structure designare sensitive to the second of the process parameters but not the firstof the process parameters.
 21. The method of claim 1, further comprisinggenerating a test structure design based on results of both of thesimulating steps such that optical measurements of one or morecharacteristics of a test structure formed on the wafer in accordancewith the test structure design are sensitive to changes in the one ormore characteristics of the test structure.
 22. A non-transitorycomputer-readable medium storing program instructions executable on acomputer system for performing a computer-implemented method forgenerating an optical model of a structure to be measured on asemiconductor wafer, wherein the computer-implemented method comprises:selecting nominal values and one or more different values of processparameters for one or more process steps used to form the structure onthe wafer; simulating one or more characteristics of the structure thatwould be formed on the wafer using the nominal values; generating aninitial model of the structure based on results of said simulating;simulating the one or more characteristics of the structure that wouldbe formed on the wafer using the one or more different values as inputto the initial model; translating results of both of the simulatingsteps into the optical model of the structure; and determiningparameterization of the optical model based on how the one or morecharacteristics of the structure vary between at least two of thenominal values and the one or more different values, wherein theselecting step, both simulating steps, the generating step, thetranslating step, and the determining step are performed without usingimages of the structure as formed on a wafer and before the structure isformed on any wafers.
 23. A system configured to generate an opticalmodel of a structure to be measured on a semiconductor wafer,comprising: an optical measurement subsystem configured to measure thestructure as formed on the wafer; and a computer subsystem configuredfor: selecting nominal values and one or more different values ofprocess parameters for one or more process steps used to form thestructure on the wafer; simulating one or more characteristics of thestructure that would be formed on the wafer using the nominal values;generating an initial model of the structure based on results of saidsimulating; simulating the one or more characteristics of the structurethat would be formed on the wafer using the one or more different valuesas input to the initial model; translating results of both of thesimulating steps into the optical model of the structure; anddetermining parameterization of the optical model based on how the oneor more characteristics of the structure vary between at least two ofthe nominal values and the one or more different values, wherein theselecting step, both simulating steps, the generating step, thetranslating step, and the determining step are performed without usingimages of the structure as formed on a wafer and before the structure isformed on any wafers.